Cationic surfactants having long (C22) mono-unsaturated tails were studied in aqueous solutions containing salt using steady and dynamic rheology. The surfactant erucyl bis(hydroxyethyl)methylammonium chloride self-assembles into giant wormlike micelles, giving rise to unusually strong viscoelasticity. Under ambient conditions, the viscosity enhancement due to surfactant exceeds a factor of 107. Some samples behave as gel-like solids at low temperatures and revert to the viscoelastic (Maxwellian) response only at higher temperatures. These samples display appreciable viscosities (>10 Pa·s) up to very high temperatures (ca. 90 °C). Salts with counterions that penetrate into the hydrophobic interior of the micelles, such as sodium salicylate, are much more efficient at promoting self-assembly than salts with nonbinding counterions, such as sodium chloride. Changing the surfactant headgroup to the more conventional trimethylammonium group reduces the viscosity at high temperatures.
Dispersions of hydrophilic fumed silica are investigated in a range of polar organic media. The silica forms stable, low-viscosity sols exhibiting shear thickening behavior in a host of liquids, including ethylene glycol and its oligomers and short-chain alcohols, such as n-propanol. In contrast, the silica flocculates into colloidal gels in other liquids, such as glycols with methyl end-caps and longer-chain alcohols. We suggest that there is a causal relationship between the hydrogen-bonding ability of the liquid and the colloidal microstructure observed. In strongly hydrogen-bonding liquids, a solvation layer is envisioned to form on the silica surface through hydrogen bonding between liquid molecules and surface silanol groups (Si-OH). This gives rise to short-range, non-DLVO repulsions ("solvation forces") which stabilize the silica particles. In contrast, in the case of liquids with limited hydrogen-bonding ability, silanols on adjacent silica particles are envisioned to interact directly by hydrogen bonding. This leads to particle flocculation and ultimately to gelation. Our study further fuels the debate regarding the existence of solvation forces in dispersions.
Small-angle neutron scattering (SANS) and rheology are used to probe the wormlike micelles formed in mixtures of a cationic (cetyl trimethylammonium tosylate, CTAT) and an anionic surfactant (sodium dodecyl benzene sulfonate, SDBS). For a fixed composition of 97/3 CTAT/SDBS, the zero-shear viscosity η 0 initially increases rapidly with surfactant concentration, but decreases beyond an intermediate concentration φ max . The solutions show a scattering peak in SANS and the height of the scattering peak also exhibits a maximum around φ max . These results are interpreted in terms of a maximum in the linear micellar contour length L h at φ max , and suggest that the hydrodynamic and electrostatic correlation lengths reach an optimal ratio at this point. For a fixed total surfactant concentration, the viscosity η 0 also reaches a maximum at an intermediate SDBS fraction. The decrease in η 0 at high SDBS fractions is interpreted in terms of the polyelectrolyte nature of the micelles and the increased chain flexibility caused by the rising ionic strength of the solutions. An alternate possibility may involve a progression from linear to branched micelles with increasing SDBS content.
Unilamellar vesicles are observed to form in aqueous solutions of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), when 5-methyl salicylic acid (5mS) is added at slightly larger than equimolar concentrations. When these vesicles are heated above a critical temperature, they transform into long, flexible wormlike micelles. In this process, the solutions switch from low-viscosity, Newtonian fluids to viscoelastic, shear-thinning fluids having much larger zero-shear viscosities (e.g., 1000-fold higher). The onset temperature for this transition increases with the concentration of 5mS at a fixed CTAB content. Small-angle neutron scattering (SANS) measurements show that the phase transition from vesicles to micelles is a continuous one, with the vesicles and micelles coexisting over a narrow range of temperatures. The tunable vesicle-to-micelle transition and the concomitant viscosity increase upon heating may have utility in a range of areas, including microfluidics, controlled release, and tertiary oil recovery.
Self-assembly in mixtures of cationic and anionic surfactants occurs synergistically because of attractive interactions between the oppositely charged headgroups. Here, such effects are exploited to obtain highly viscoelastic fluids at low total surfactant concentration. The systems considered are mixtures of the C18-tailed anionic surfactant, sodium oleate (NaOA), and cationic surfactants from the trimethylammonium bromide family (C n TAB). In particular, mixtures of NaOA and C8TAB show remarkably high viscosities: for 3% surfactant, the zero-shear viscosity η 0 peaks at ca. 1800 Pa·s for a weight ratio of 70/30 NaOA/C8TAB. The high viscosities reflect the growth of giant, entangled wormlike micelles in the solutions. Mixtures of NaOA with a shorter-chain analogue (C6TAB) have much lower viscosities, indicating a weak micellar growth and hence a weak attraction between the surfactants. On the other hand, increasing the C n TAB tail length to n = 10 or 12 leads to much stronger interactions between these surfactants and NaOA. Consequently, both micellar and bilayer structures are formed in these mixtures, and the samples separate into two or more phases over a wide composition range. Thus, the synergistic growth of wormlike micelles in cationic/anionic mixtures is maximized when there is an optimal asymmetry in the surfactant tail lengths.
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